Thermal coupling links in distillation are known to reduce the overall costs of a configuration on a plant, owing to simultaneous reduction in capital and operating costs. Referring to FIG. 1, a fully thermally coupled three-component Petlyuk configuration with thermal coupling links at submixtures AB and BC is shown. In all the figures of this patent application, unfilled circles denote reboilers 101, while filled circles denote condensers 103. Furthermore, the configuration of FIG. 1 is referred to as the TC-TC configuration. The first and second ‘TC’ respectively denote the thermal coupling links at submixtures AB and BC.
Despite its potential to significantly reduce the overall costs, the TC-TC configuration has seen limited industrial application. One reason for this is the operability issue that accompanies the TC-TC configuration. In FIG. 1, vapor AB is withdrawn from the top of a first column 105, and fed to a second column 107. Still referring to FIG. 1, this requires the pressure at the top of section 1b to be greater than that at the bottom of section 2a (assuming compressors are not used in the transfer line). Further, vapor BC is withdrawn from the top of section 2d, and fed to the bottom of column 1. This requires the pressure at the top of section 2d to be greater than that at the bottom of section 1c. Such conflicting pressure requirements in the two distillation columns bring in operational complications to the TC-TC configuration. To overcome these operability issues, Agrawal and Fidkowski suggested the use of configurations in FIGS. 2a and 2b, which are thermodynamically equivalent to the TC-TC configuration. In the configurations of FIGS. 2a and 2b, the pressure in one column can be uniformly maintained greater than the other column, which simplifies some of the major operational complications of the TC-TC configuration.
For further savings in plant space and capital costs, the TC-TC configuration can be incorporated into a single shell, popularly called the dividing wall column, as shown in FIG. 3. This configuration shall be henceforth referred to as the TC-TC column. A naming system has been adopted where TC-TC configuration refers to the two-column configuration shown in FIG. 1, and TC-TC column refers to the one column system with a vertical partition as shown in FIG. 3. Also, it is to be noted that, later in the disclosure herein, the skeleton partitioning arrangement/structure of FIG. 3 is referred to by the same name (TC-TC column), even when it is used for separating four or higher component feeds. In the case of multicomponent separations using TC-TC column, the submixtures transferred at the thermal couplings will differ from what is shown in FIG. 3. Further, for convenience, the different parts of dividing wall columns in the paper are shaded and named distinctly to represent different zones. For example, the TC-TC column of FIG. 3 is divided into four zones, namely ZT, ZB and the vertical partition's zones Z1 and Z2.
Although the dividing wall column was introduced by Wright as early as 1949, the first industrial application of this column did not happen until the late 1980s. Since then, the use of multicomponent dividing wall columns has seen a rapid increase in several industrial applications. Updates on the recent developments in dividing wall columns can be found in the works of Aspiron and Kaibel, Dejanovic et al. and Yildirim et al.
Though the TC-TC column of FIG. 3 offers ample opportunity to reduce overall costs, it suffers from somewhat similar operability issues (related to pressure) as the TC-TC configuration of FIG. 1. The pressure drop in the TC-TC column is an important consideration for its onsite operation. In the TC-TC column, the pressure drop in the two parallel zones, Z1 and Z2 of the vertical partition, on either side of the vertical partition, are constrained to be equal. Subject to this constraint and the mechanical resistances in the vertical partition's Z1 and Z2 zones, there is a natural uncontrolled split of the vapor ascending from the zone ZB into the vertical partition's zones Z1 and Z2. This uncontrolled split implies that the relative vapor flowrates in zones Z1 and Z2 of the vertical partition cannot be manipulated during operation. Though methods to address the control of the vapor split issue during the design and dimensioning phase of the TC-TC column have been proposed, none exists for application during online operation. This vapor split can significantly affect the product purities, total annualized costs, and has implications on how far the TC-TC column is away from its optimal operation. Though the liquid split at the top of the vertical partition also can have similar effects, it is generally well-controlled during operation, using collectors and distributors. Further, the operable versions of the TC-TC configuration shown in FIGS. 2a and 2b also simplify to the same dividing wall column arrangement of FIG. 3. Hence, the operational advantages in the configurations of FIGS. 2a and 2b over the TC-TC configuration are not translated to their dividing wall versions.
Alternate dividing wall columns, as shown in FIGS. 4a and 4b, operating in the side-stripper and side-rectifier modes have been proposed in the literature. In these dividing wall columns, the split of vapor to the two parallel zones Z1 and Z2 of the vertical partition can be controlled using the reboilers 101 and condensers 103. Also, these dividing wall columns are often thermodynamically more efficient than the TC-TC column. However, they do not always retain the same minimum heat duty requirements as the TC-TC column for all feed conditions.